1. Field of the Invention
This invention relates to a process for applying a porous metal coating to a metal substrate or a portion thereof and more particularly, a porous coating of a reactive metal such as titanium or titanium base alloy applied to a substrate of similar base material. The invention also relates to a mold for use in the process. The process and mold are especially, although not exclusively, useful for making medical prostheses.
2. Brief Description of the Prior Art
Prior art descriptions and practices applicable to titanium and other reactive metals utilize either flame spraying or adhesive bonding, followed by high temperature sintering, to produce a porous coating on a metal substrate. While these former processes are useful to produce porous coated products, the products are subject to substantial dimensional variation. Moreover, the thickness of the porous coating is also subject to considerable variation due to application techniques. Furthermore, with reactive metal systems, contamination of the applied coating by the adhesive is a definite problem.
The desirability of producing porous surfaces on medical prosthetic devices is well documented, for example in Pilliar U.S. Pat. No. 3,855,638. Hahn U.S. Pat. No. 3,605,123 and Kafesjian et al. U.S. Pat. No. 4,017,911 describe methods of producing such porous coatings. Further, Tronzo U.S. Pat. No. 3,808,606 also describes medical devices having porous surfaces.
The various medical factors involved in bone or tissue ingrowth or even those involved in bone cement adhesion are well-known to those in the art and need not be discussed in detail herein. However, the medical community has clearly indicated that control of the pore size in porous coatings is highly desirable and dimensional control absolutely necessary for satisfactory prosthetic devices. Additionally, careful control of chemistry and the elimination of sources of contamination are also recognized to be essential for implanted prosthetic devices.
The known flame spray method as illustrated by Hahn Pat. No. 3,605,123 of applying a coating to a metal substrate has two major limitations. The control of pore size and volume is difficult due to the very nature of this method and the control of the dimensional characteristics of the coating is also complex.
The use of adhesives as described in U.S. Pat. Nos. 3,808,606 and 4,017,911 overcomes the porosity problems associated with the flame spray method; however, products made with adhesives also are subject to considerable dimensional variation. As important, the use of glue or other adhesives on reactive metals such as titanium frequently involves contamination of the pore-producing media, i.e. powder, as well as contamination of the substrate itself.
Tronzo U.S. Pat. No. 3,840,904 discloses an acetabular hip prosthesis and suggests the desirability of anchoring a prosthesis using a porous coating. However, the patent does not disclose any method or apparatus for applying the coating to a substrate or to a portion thereof.